Subsea trenching operations are routinely performed to provide protection for pipelines, umbilicals and power cables. The increase in offshore wind farm developments and the focus on environmental impact from new regulatory bodies and the public have focused attention on trenching operations. This paper reviews the methods of trenching routinely used and their impact on the seabed. Two case studies are presented showing the actual and modelled effects of trenching operations. Consideration is given on methods that could be used for real-time monitoring of trenching operations.
There is frequently a requirement to bury pipelines and cables into seabed sediments to protect them from external threats. A variety of trenching techniques are used that are dependent on the seabed sediments present and the nature of the product being trenched. The trenching operations can have a potentially adverse effect on the seabed environmental conditions, through lifting sediment particles into the water column and disturbing the seabed surface sediments. This paper reviews the operation of the different trenching techniques available, including jetting, ploughing and mechanical cutting tools. It then assesses and discusses the impact of these techniques on the seabed environment. This is then placed in the context of naturally occurring events, such as sediment mobility, and anthropogenic activities, such as fishing and aggregate dredging.
2. Trenching Operations – Method and Impacts
Submarine cables, umbilicals and pipelines must be protected from damage, which could be caused by accidental impact from ships anchors or trawling activities. Fatigue of products can also occur if the seabed around them is scoured by wave or tidal currents. To mitigate against these effects, pipelines, umbilicals and cables are routinely buried beneath the seabed, or lowered into an open trench that is deep enough to provide protection (Machin, 2000).